Source unit for producing ionized gas



March 19, 1957 L. RUBY ET AL 2,786,143

SOURCE UNIT FOR PRODUCING IONIZED GAS Filed Aug. 16, 1955 I 0.0 POWER /4/ SUPPLY Q Q 6 I 40 PULSE 1 FORMING: NETWORKJ) Q B *3 1 l .62.

0' TRIGGER GENERATOR ATTORNEY.

SOURCE UNIT FOR PRODUCING'IONIZED GAS Lawrence Ruby, Berkeley, Richard B. Crawford, Walnut Creek, and Wing G. Pon, Oakland, Calif.,, assignors to the United States of Americarepresented. by the United States Atomic Energy Commission Application August 16, 1955, SerialNo. 528,852

' 11 Claims.- (Cl. 250-419) The present invention relates to apparatus for producing a space charge neutralized ion cloud, and more specifically to a source unit for providing-a highdensity plasma for use in manifold utilitarian apparatus.

The term plasma will herein signify an electrically neutral ion cloud. Such a plasma may be produced and utilized in the operation of accelerators, calutrons, ion sources, current conducting devices and various other apparatus utilizing electrical discharge through gases.

Apparatus employed in general practice to produce ionization generally are associated with coincident neutral gas production and electron beams; and many require cumbersome cooling apparatus, auxiliary magnetic fields, charged particle accelerating means, external gas supplies, and separate anode and cathode arc electrodes. In addition, conventional ionizing devices are; not readily adjustable for accurately controlling and regulatingrapid repetitive plasma production forextensive operation.

Many of the diificulties encountered in conventional plasma generating apparatus are eliminated in the present invention. In the source unit of the'invention a novel plasma emitter, comprising a plurality of interleaved occluded-gas metallic discs and insulating spacers, is employed to perform the function of the are electrodes and external gas supply utilized in conventional sources. Plasma generation is accomplished by establishing an electrical arc discharge between peripheral surfaces of the metallic discs for a specific interval, i. e., for the duration of the discharge, duringwhich time occluded gas is released and instantly ionized by the arc,. and therefore a dense plasma is generated with negligible coincident. contamination by neutral gas molecules-or associatedelectron beams. Moreover the magnitude, pulserepetition rate, and duration of plasma production can be regulated by appropriate electrical circuitry utilized'to establish the arc discharge.

It is therefore an object of theinvention to provide a new and improved source unit for producing a plasma.

One object of the invention is'to provide a source unit that will produce a high plasma yield.

Another object of the invention is to provide a source unit for producing a plasma that can be repetitiously operated at selected intervals for specific. durations of plasma generation.

An additional object of the invention is to provide a pulsed source unit for producing a plasma without neutral gas contamination between pulses, thereby providing a very high ionization efficiency.

i A further object of the invention is to provide a source unit for producing a plasma which eliminates the necessity of an external gas supply. j

An important object of theinvention is to provide" a source unit for-producing a plasma utilizing a novel plasma emitter comprised of interleaved occluded-gas metallic discs and insulating spacers.

Still another object of the invention is Lto. provide a source unitifor producing a plasma. in which the rate of plasma generation can be regualted.

atent 2,786,143 Patented. Mar. 19, 1957 lowing specification taken in conjunction. with the. ac

companying drawing, of which:

Figure 1 is a perspective view ofthe plasma. emitter;

Figure 2 is a longitudinal cross sectional view. of a preferred embodiment of the invention. showing the C166? trical components diagrammatically; and

Figure 3 'is a cross sectional view taken along plane 3-3 of Fig. 2 indicatingthe direction of plasma emission with arrows.

In brief, therefore, the source unit of the invention utilizes in combination and arranged in an evacuated sys.-. tem, a. plasma emitter comprising a plurality of interleaved occluded-gas metallic and dielectric discs secured between clamping electrodes. Electrical pulsing means is coupled to the plasma emitter to establish an electrical arc discharge between peripheral surfaces of saidmetallic discs from which occluded gas is released and ionized.

More particularly, referring to the accompanying drawing and specifically to Fig. 1, the plasma emitter indicated generally at 11 includes a plurality of spaced metallic discslZ in which aselected gaseous material is occluded. The metallic discs should be constructed of material, having the ability to absorb large quantities of gaseous material, such as one of the transition metals, e. 'g., titanium, tantalum, palladium, zirconium and the like. Titanium is a metal preferred for this purpose since. it is capable of absorbing extremely large amounts of a gaseous material, particularly the hydrogen isotopes, and will retain such gas even in very high vacuum systems until such time that sufficient heat is imparted thereto as'by an electrical are or thelike, as described more fully hereinafter.

The metallic discs 12 may be prepared by outgassing at several hundred degrees centigrade (red heat) in an evacuated system as by means to apply a high current thereto for a period of time adequate toremove substantially all the undesirable gases therein. The metallic discs are then allowed to cool slowly for approximately one hour during which time the selected gaseous material to be occluded therein is admitted-at. atmospheric pressure to contact the discs. As the discs cool they absorb large quantities of the selected gaseous material. This absorption is accompanied by large volume expansion (10 to 15%) and by embrittlement. The evidence is quite conclusive that in the transition metals, the occluded gas is not only found in lattice rifts, as is the case in other gas-absorbing substances, but also is distributed regularly within the lattice structure. The gas; is contained in the atomic form, atleast partially ionized and, in fact, it is believed to be most probably in the metallic state as suggested by the fact that although the size of a hydrogen atom is small enough to require no expansion in the transition metal lattice to contain it, it has been observed to reach an expansion of 11%. Calculations have shown that metallic hydrogen would be stable relative to covalent hydrogen at pressures not less than 25x10 atmospheres, and such an internal pressure in metal such as palladium would require a volume increase of about 10%.

It will be noted here that titanium at low temperatures has thelargest known capacity to absorb hydrogen and, in addition, the volume concentration .of hy-drogenin titanium is greater than in any other form in which hydrogen is known to exist, and therefore the metallic discs 12 are preferably constructed of; titanium;

- Inorder to insure uniformity of gap spacing, insulators 13 of material such as mica are disposed between eaehmetallic disc '12. The threshold voltage for breakdown across the peripheral surface of the insulators 13 is, of course, dependent on the thickness thereof. Care should be taken to. avoid extremely thin insulators since they frequently puncture from sparking through the body instead of across the surface.

Thefmetallic discs 12 and insulators'13 are apertured toform a bore 14 longitudinally through the plasma emitter 11 for purposes of inserting a rod-like retainer 16of insulating material to adapt the plasma emitter 11 for support, as shown in Figs. 2 and 3'." v

The hereinbefore mentioned clamping electrodes, preferably' comprise cylindrical electrodes 17 and 18 constructed of material such as stainless steel, or other electrically conducting materials .preferably pervious to a magnetic field if the device of the invention is to be utilized in a magnetic field, e. g., in'a cyclotron or the like. For purposes of compactness, electrodes 17 and 18 are disposed in parallel relationship so as to support the plasma emitter 11 perpendicularly therebetween at the lower extremities thereof. It will be appreciated that other arrangements may be employed, e. g., the plasma emitter 11 could be supported'colinearly between electrodes 17 and 18, depending on the particular application for which the source unit is to be utilized.

As shown in Figs. 2 and 3, the plasma emitter 11 is supported perpendicularly between the lower terminations of electrodes 17 and 18; The terminal extremities of insulating retainer- 16 are disposed in bores 19 and 21, respectively, extending a short distance inward from the proximal, surfaces of electrodes 17 and 18 at the lower extremities thereof.

' For immobilizing electrodes 17 and 18, two spaced cylindrical ceramic standoffs 22 and 23, aligned in spaced parallel relationship with the plasma emitter 11, are provided with threaded fasteners or screws 24 and 26 extending through suitable bores 27 and .28 in electrodes 17 and 18 and seated in threaded bores 29 and 31 in standoffs 22 and 23, respectively.

"A metal plate 32 serves as a convenient support for the source unit described and renders it attachable in a hermetic fashion to such apparatus as includes an evacuated region, e. g., an accelerator, in which the source unit maybe utilized, in which instance plate 32 serves also as a closure. Plate 32 is not necessary in the event the source unit is placed directly in the evacuated region of apparatus whichincludes conventional housing structure in which it will be utilized. For thefirst-meutioned application and shown in Figs. 2 and 3, the plate 32 may be provided with an annular groove 33 in the bottom surface thereof for receiving a vacuum sealing element, and bolt holes 34 are provided at the outer edge to adapt the plate for attachment to other apparatus. The upper termination of electrode 17 passes through an aperture 36 in pl'ate 32 and is hermetically insulated therefrom by a short, cylindrical metal-to-glass seal 37, as is known in the art. Electrode 18'extends through aperture 38 in plate'32 to'which it is secured as by nut 39. The portion of electrode 18 proximal the lower surface of the plate 32 may be sealed as by soldering thereto to provide a vacuum tight seal.-

The hereinbefore-mentioned electrical pulsing means to energize the source unit may comprise an electrical circuit 40 coupled to clamping electrodes 17 and 18 to provide a pulse line voltage across the plasma emitter 11. As shown in Fig. 2, the circuit 40 includes a D. C. power supply 41 having its negative terminal connected through a conveniently designed pulse forming network 42 to electrode 18, which is allowed to float electrically. The positive terminal of the power supply 41 is connected through'a charging resistor 43 to the'pulse forming net: work 42' and thence througl'ra terminating resistor 44 to the anode of a thyratron 46. The cathode of thyratron 46 is connected to electrode 17. A trigger generator 47 is provided'to actuate thyratron 46 through an 4 isolating pulse transformer 48; one terminal of the secondary of transformer 48 is connected to the grid of thyratron 46 and the other terminal is connected to a juncture 49 located between the cathode of thyratron 46 and electrode 17. The trigger generator 47 is of any conventional design and may be adapted for application of an external triggering signal which may be supplied by control apparatus of the device with which the source 1 unit is employed. Thus the source unit may be actuated very precisely-as required in the operation of the associated apparatus. Y

In operating the source unit, necessarily in an evacuated 7 system, thyratron 46 is actuated by a positive pulse genreleased and instantly ionized by the are thereby establishing plasma generation.

It should be mentioned that gas emission and simultaneous ionization is confined exclusively to the duration of the electrical arcing, as determined by the electrical characteristics of the circuit energizing the source unit, and will cease upon discontinuance thereof. In addition, only the localized peripheral surfaces of the metallic discs 12 directly in contact with the electrical arcs are heated sufficiently to cause gas emission, and thus essentially all of the gas emitted will be instantly ionized by the same are that caused emission. Therefore, substantially no neutral gas contamination is introduced into the system either during plasma generation or between pulses with the result that exceedingly high ionizations efficiencies can be obtained by the source unit of the invention.

It will be found that a magnitude of plasma, dependent on the pulse line voltage, can be repetitiously produced at a pulse repetition rate regulated by the trigger generator 47 for pulse durations controlled by the characteristics of the pulse forming network 42 which determines the time interval required to completely discharge the pulse line voltage across the plasma emitter 11. Therefore, the magnitude, pulse repetition rate, and duration of plasma generation, from the peripheral surfaces of the occludedgas metallic disc 12, can be readily controlled by proper selection and adjustment of the electrical components of circuit 40. For example, with a conventional collector electrode disposed to extract an ion beam from the plasma generated by the source unit of the invention illustrated in Figs. 1 and 2 wherein conventional components were utilized to provide a trigger voltage to thyratron 46 and to develop a 9 kv. pulse line voltage which appeared across plasma emitter 11 and initiated a ampere discharge of 150 microseconds duration, an ion beam current of approximately 4.5 amperes was collected on said collector have a large quantity of dense ionized gas available for use. For example, the source unit can be utilized to supersede the function of the 'arc' electrodes and external gas supply employed in conventional ion sources. Also, the source unit could be utilized to provide a controlled source ,of electrons for an electron gun and thus advantageously replace the conventional filament element which has a much shorter operating life than the source unit. It is also feasible that the plasma generated by the source unit of the invention, being readily controllable, could provide the conducting medium for higher voltage switching than is obtainable by conventional thyratrons or ignitrons.

While the invention has been described with reference to a preferred embodiment, it will be apparent to those skilled in the art that numerous variations and modifications may be made within the spirit and scope of the invention and thus it is not intended to limit the invention except as defined in the following claims.

What is claimed is:

1. A source unit for producing ionized gas in an evacuated space comprising in combination a plasma emitter of interleaved occluded-gas metallic and dielectric discs, and electrical means for establishing an electrical discharge between peripheral surfaces of said metallic discs to release and ionize gas occluded therein.

2. A source unit for producing ionized gas in an evacuated region comprising in combination a plasma emitter constructed of a plurality of interleaved metallic and dielectric discs having a selected gaseous material occluded in at least the peripheral surfaces of said metallic discs, and electrical arcing means for releasing and ionizing said gaseous material.

3. A source unit for producing a plasma in an evacuated region comprising a plasma emitter constructed of interleaved metallic and dielectric discs having a selected gaseous material occluded in said metallic discs, an insulating rod disposed longitudinally through said discs for supporting the same, and electrical means for establishing an electrical are discharge between peripheral surfaces of said metallic discs.

4. A source unit for producing a plasma in a low pressure region comprising in combination a plasma emitter constructed of a plurality of interleaved metallic and dielectric discs having a selected gaseous material occluded in the peripheral surfaces of said metallic discs, an insulating rod disposed longitudinally through discs for supporting the same, spaced clamping electrodes secured to the extremities of said plasma emitter for supporting the same, and electrical means connecting to said electrodes for establishing an electrical arc discharge between peripheral surfaces of said metallic discs.

5. A source unit as described in claim 4 wherein said electrical means comprises an electrical pulse generator.

6. A source unit for producing a plasma in an evacuated region comprising in combination a plasma emitter constructed of interleaved occluded-gas metallic and dielectric discs, an insulating rod for supporting said discs and disposed longitudinally therethrough, clamping electrodes secured to the terminations of said plasma emitter for supporting the same perpendicularly therebetween, and an electrical pulse generator for establishing an electrical discharge across said plasma emitter and thereby releasing and ionizing occluded gas from peripheral surfaces of said metallic discs.

7. A source unit as described in claim 6 wherein said occluded-gas metallic discs comprise hydrogenated metallic discs.

8. A source unit as described in claim 6 wherein said occluded-gas metallic discs comprise hydrogenated titanium discs.

9. A source unit as described in claim 6 wherein said dielectric discs comprise thin mica discs.

10. A source unit for producing a plasma in an evacuated region comprising in combination a cylindrical plasma emitter constructed of interleaved hydrogenated metallic and dielectric discs, a cylindrical insulating rod disposed longitudinally through said plasma emitter for supporting said discs, magneticallypermeable and electrically conductive clamping electrodes secured to the terminations of said plasma emitter for supporting the same perpendicularly therebetween, ceramic standoffs secured between said electrodes and disposed in spaced parallel relationship with said plasma emitter, and an electrical pulse generator for establishing an electrical discharge across the peripheral surfaces of said plasma emitter and thereby releasing and ionizing occluded gas from peripheral surfaces of said metallic discs.

11. A plasma source for use in an evacuated space comprising in combination a cylindrical plasma emitter constructed of interleaved hydrogenated titanium and mica discs, a cylindrical insulating rod disposed longitudinally through said discs for supporting the same, cylindrical clamping electrodes secured to the extremities of said plasma emitter for supporting the same perpendicularly therebetween, cylindrical ceramic standofis secured between said electrodes and disposed in spaced parallel relationship with said plasma emitter, and an electrical pulse generator for establishing an electrical discharge across the peripheral surfaces of said plasma emitter and thereby releasing and ionizing hydrogen isotopic gas from peripheral surfaces of said hydrogenated titanium discs.

References Cited in the file of this patent UNITED STATES PATENTS 2,211,668 Penning Aug. 13, 1940 2,240,914 Schutze May 6, 1941 2,489,436 Salisbury Nov. 29, 1949 

1. A SOURCE UNIT FOR PRODUCING IONIZED GAS IN AN EVACUATED SPACE COMPRISING IN COMBINATION A PLASMA EMITTER OF INTERLEAVED OCCLUDED-GAS METALLIC AND DIELECTRIC DISCS, AND ELECTRICAL MEANS FOR ESTABLISHING AN ELECTRICAL DISCHARGE BETWEEN PERIPHERAL SURFACE OF SAID METALLIC DISCS TO RELEASE AND IONIZE GAS OCCLUDED THEREIN. 